Electric furnace



Patented July 9, 1935 i UNITED STATES 2,007,605 Y ELECTRIC FURNACEAlbert H. Heyroth and James I. Miller, Niagara Falls, N. Y., assignorsto The Global' Corporation, New York Application January 16,

6 Claims.

This invention relates to the manufacture of ceramic articles andparticularly to the annealing of such articles and to the apparatus forcarrying out the process.

The object of the invention is to provide a new and improved process forremoving the irregularities, such as warping, produced during the priorsteps in the manufacture of long slender ceramic articles and to providenew and improved apparatus for accomplishing this purpose. Other objectsof the invention will become apparent from the following specificationand claims.

Long, slender, ceramic articles, such as hollow insulators forthermocouple wires and thermometer tubes, should be straight and uniformin their finished state, but inasmuch as the methods cmployed heretoforehave produced a great many Warped and irregularly shaped articles it hasbeen necessary to reject a large percentage of the articles. made and touse only the ones that came through the process in a satisfactorycondition. There has been no way of reclaiming articles that becomedistorted.

By our process we are enabled to reclaim the majority of articles thatwould be rejected as unsuitable if made according to the prior methods.

Some of the principal problems that we have solved are the shielding ofthe ceramic objects to be treated from direct radiation, and the supportof the ceramic objects in such a manner that they may be inserted andremoved from the furnace simultaneously without seriously affecting thetemperature conditions of the furnace.

The furnace which we have designed to meet these conditions isillustrated by the accompanying drawing in which:

' Figure 1 is a vertical section of the electric annealing furnace in aplane at right angles to the resistor rods used for heating the furnace;and

Figure 2 is a section on the line 2-2 of Figure l.

Referring to the drawing in detail, a heating chamber 2 is provided witha plurality of rigid resistor rods 3 which consist mainly of siliconcarbide. These resistor rods are mounted at the sides of the heatingchamber which is permanently closed on all sides excepting a portion ofthe top wall Where there is a removable cover piece 4. There may also bea small door near the base for the removal of dbris from time to time.The cover piece 4 is used to support a large number of ceramic objects 5which are held in holes I0 made in the lower portion of the cover. Thefurnace has a number of similar cover pieces 4. Cover pieces not in useon the furnace are loaded with a plurality of ceramic articles 5 byinserting and Niagara. Falls, N. Y., va. corporation of 1931, Serial N0.509,136

holding them in the holes provided for the purpose. The cover piece withits load is placed in such a manner as to bring the ceramic articles inthe central portion of the heating chamber and close up the furnace asillustrated in the drawing. The ceramic articles hang in a position inwhich the action of gravity tends to straighten them out at hightemperatures. The ceramic articles experience a gradual change intemperature on introduction to the furnace as they are shielded fromdirect radiation emitted by the resistor rods 3.

The shielding mechanism disclosed in the drawing consists of two banksof cylindrical rods or bars 5 which rest in a position inclined slightlyto the vertical and so that they loosely Contact with each other. Theserods are preferably made of silicon carbide because rods of thismaterial, when properly made, retain a large part of their mechanicalstrength even up to a dazzling white heat. This is particularly true ofrods made of recrystallized silicon carbide. While some radiationpenetrates between rods 6 it is so scattered by reflection from thetangential surfaces of the rods 6 that the articles 5 are not subjectedto appreciable radiation from the resistor rods 3. 'Ihis result followsalso from the fact that the resistor rods 3 are at right angles to thescreening rods 6.

The resistors 3 are made of reorystallized or bonded silicon carbide.They extend completely across the heating chamber, the end portionsbeing supported in conical recesses 'l which extend slightly into thefurnace wail. Mechanical support and electrical engagement are obtainedby butt-end engagement with resiliently supported water-cooled terminals(not shown) such as are disclosed in the United States Patent to Shaw, No. 1,742,286, January 7, 1930.

The cover 4 and inner section of the furnace wall 8 are made of hightemperature refractory, such as fused alumina bricks or silicon carbidebricks. The high temperature refractory section of the wall issurrounded by a wall section made of insulating bricks 9 composed ofporous clay refractory such as is described in the United States patentto Hartmann, No. 1,545,559, patented July 14, 1925. The porous clayrefractory section is surrounded by another insulating section ll made,for example, of Silo-cel bricks. An outer section l2 of low temperatureinsulating material such as magnesia or asbestos can be provided tofurther reduce the heat losses. This material is contained within anouter casing I3.

On account of the large heat capacity of the banks of massive rods t andthe refractory walls 8 (as well Ias the extensive provisions forinsulation) the furnace is very economical of power and emcient inoperation.

Our furnace is especially adapted t'o the mass production of ceramicarticles which require uni- Vform heat treatment under conditions whichextended lengths as showny inthe drawing. After the heat treatment iscompleted, such extended bodies are cut into suitable lengths. Inthiswayit is possible to manufacture short objects of regular shape and sightlyappearance. These results would be difcult to obtainv if these shortrobjects were heat treated in short lengths.

The silicon carbide shields or bales 6 have many advantages in additionto those already mentioned.4 |'ihey provide a larger radiating surfacethan would beprovidedby baflies composed of bricks or tiles. They alsopermit close temperature control of the objects 5 Vby reason of the goodthermal conductivity of,y silicon carbide andV its high ernissivity.VThey also provide a more uniform distribution of vthe. heat from' theresistors and are readily replaced. r

'The furnace which we have described is especially easy to operate fromthe fact that when it is opened itV throws the radiation which escapeson removal of the cover in an upward direction where it does not disturbworkmen in the Vicinity.

i The ceramic objects to be heat treated can therefore be inserted orremoved with a minimum of inconvenience. Deb-ris which falls fromobjects to be heat treated collects on thev floor of the heating chamberand may be removed through a door iii indicated in Figure 1.

Thev method which we have described may be applied 'to any kind ofceramic' object which is intended to have a substantiallyuniform crosslsection. Thermometer tubes and baked clay cyl- 45` inders may be citedVas examples. Clay bonded articles are vitried previously to thestraightening process. The straightening temperature is carried to theinitial softening point of the'glass or clay bond. AVVinstead of Vglassor'clay other bonding materials of simpler vcharacter may be used suchas calcium fluorideV or other alkaline earth halide, aluminum fluoride,etc. Binary mixtures may also be used as bonds,"such as a mixture oflime, 25-35 per cent and titanium oxide, 65-75 per cent. Bonds of thischaracter are particularly resistant to reducingatmospheresy attemperatures in the neighbourhood of 1,300 C.

The advantages of our method of heat treatment of cylindrical ceramicobjectsare particularly obvious when this method'is compared with theusual method of heat'treatment .of such objects while they are supportedin a horizontal `position in contactv with horizontally disposedhearths.A En the case of. our' method, on the contrary, ,the portions ofthe ceramic objects which are to be later used are supported in isolatedVpositions where they are heated only by radiation and their'sides arein contact with gaseous material only, thereby eliminating the liabilityof injury or disgurementof the objects during insertion or removal.

We claim:

l. A furnace for the heat treatment of ceramic objects comprising aheating chamber, electrical heating resistors within said chamber, aremovable cover from which objects to be heat treated are supported invertically downward position by attachment of their upper ends to thecovenand a screen of refractory rods interposed between the electricalresistors and the ceramic objects under heat treatment.

2. A furnace for the heat treatment of ceramic lobjects comprising aheating chamber, electrical heating resistors Within said heatingchamber, a removable cover from the lowerV surface of which extendobjects to be heat treated while secured to said cover to extend intovertically downward position in said heating chamber, a bank ofrefractory bars of high thermal conductivity andhigh emissivity placedbetween said resistors and the objects to be heat treated, a v

wall 'of high temperature refractory surrounding said heating chamber,and Walls of insulating material surrounding said high temperaturerefractory. Y Y

3. A furnace 'for the heat treatment of ceramic objectscomprising aheating chamber, electrical heating resistors horizontally disposedadjacent to sido walls of said heating chamber, aremovable coverofhighly refractory material to which are attached ceramic objects to beheat treated i in a vertical position in said heating chamber; a bank ofrefractory bars of high thermal conductivity and'high emissivity placedbetween said resistors and the objects to be heat treated, a wall ofhigh temperature refractory surrounding said heating chamber, and wallsof insulating material surrounding said high temperaturek refractory.

4. In an electricy furnace, a furnace chamber, a rod shaped resistorextending along a side wall of the said chamber, and a seriesoffrefractory rods of high thermal conductivity interposed between theresistor and the ware being treated in the furnace, the said rods beingadapted to shield the warefrom' the direct radiation of the resistor andbeing positioned so that their principal axes are Vin a directionsubstantially atright angles tothe principal axis of the resistor, therods being adapted to receive heat from the resistor and reradi'ate itfrom Van area substantially wider than the lateral area of the resistor.

5. In an electric furnace, a furnace chamber, a rod-shaped resistorextending horizontally along a side wall of the said chamber, and aseries of refractory rods of substantially circular cross sectionadjoining each other and interposed between the resistor and the warebeing treated in the furnace, the said rods being adapted to shield theware from direct radiation from the resistor and being in an uprightVposition so as to reradiate theV heat from the resistor from an areasubstantially wider than the lateral area of the resistor.

6. The furnace described in claim 5, in which thev rods shielding theware from'the resistor are composed of silicon carbide.

ALBERT n. HEYROTH. JAMES I. MILLER.

